The problem with using oil to cool the plant and the cooling the oil with a heat exchanger is that you are introducing a bottleneck into your system in that you are relying on convection to transfer heat from the hot surface to the heat exchanger through the oil.

Apart from anything else this greatly complicates the analysis and control of the problem. a big risk is that the average temperature of the oils could be within acceptable limits but you start to vapourise it at the hot surface which drastically drops the heat transfer rate and is difficult to detect or do anything about.

Using forced convection by circulating the oil itself greatly simplifies the problem and gives you a lot more direct control over the rate of heat transfer simply by increasing the flow rate.

As you already have a cubic foot of oil in the tank the extra you would need to fill piping and a radiator is not huge, bearing in mind also that water used for cooling usually requires additives to prevent corrosion etc which in themselves may not be cheap.

The problem with using oil to cool the plant and the cooling the oil with a heat exchanger is that you are introducing a bottleneck into your system in that you are relying on convection to transfer heat from the hot surface to the heat exchanger through the oil.

Apart from anything else this greatly complicates the analysis and control of the problem. A big risk is that the average temperature of the oil could be within acceptable limits but you start to vapourise it at the hot surface which drastically drops the heat transfer rate and is difficult to detect or do anything about.

In this sort of context cost effectiveness can be as much about a simple and conservative solution which can be made to work with a fairly modest design input and this approach often ends up cheaper in the end than something which takes a lot of development and modelling to get working effectively.

Using forced convection by circulating the oil itself greatly simplifies the problem and gives you a lot more direct control over the rate of heat transfer simply by increasing the flow rate.

As you already have a cubic foot of oil in the tank the extra you would need to fill piping and a radiator is not huge, bearing in mind also that water used for cooling usually requires additives to prevent corrosion etc which in themselves may not be cheap.